In the field of lithographic printing, a metal substrate is widely used as the substrate for a lithographic printing plate support used in a lithographic printing plate precursor for the production of a lithographic printing plate. In particular, aluminum is known to form an oxide film by supplying direct current using the aluminum as an anode in an acidic solution and this metal is advantageous in various points, that is, a treatment generally known as an alumite treatment can be applied and moreover, the metal is lightweight and inexpensive. When an alumite treatment is applied to the aluminum surface, alumina having high acid resistance or high hardness as compared with metal aluminum is formed as the oxide film and a large number of small holes called pores are regularly produced in the film structure to greatly increase the surface area according to BET method (gas adsorption method). Therefore, the alumite treatment is advantageous in that improvements such as improvement of hydrophilicity of a lithographic printing plate support and improvement of adhesive strength at the time of forming a coating film can be attained, and when a printing plate is produced, both excellent staining resistance (in the present invention, referred to as “difficult staining”) and excellent press life can be obtained.
In recent years, a so-called heat-mode CTP lithographic printing plate precursor (hereinafter simply referred to as a “heat-mode lithographic printing plate precursor”) is attracting attention, where an image can be formed by exposure with light in the region from near infrared to infrared ray, particularly, a printing plate can be produced directly from digital data of a computer or the like by recording an image while utilizing the heat generated upon light irradiation with a laser having light emission in that region.
In this lithographic printing plate precursor, the laser light irradiated for drawing an image is converted into heat by a light-to-heat conversion material or the like contained in the photosensitive layer and the heat generated is used for changing the solubility of the photosensitive layer in a developer or causing thermal decomposition or, due to abrupt heating, explosive expansion and removal (ablation) of the photosensitive layer. When aluminum is used as the support of the heat-mode lithographic printing plate precursor, high heat conductivity of the aluminum allows radiation of the generated heat toward the support side to result in the loss of the generated heat and this is one of causes for the reduction in the sensitivity of the lithographic printing plate precursor. In other words, when the heat insulating property on the surface of a lithographic printing plate support is enhanced and the radiation of heat generated in the photosensitive layer can be minimized, it is estimated that the sensitivity of a lithographic printing plate precursor can be elevated.
A technique of elevating the sensitivity by using an organic material having a low heat conductivity, such as PET, for the support has being studied. However, such materials are low in the hydrophilicity as compared with metal materials and absorb moisture during the printing to deteriorate the dimensional precision and therefore, these materials cannot be used at present for high-level printing such as color printing and high-precision printing.
Accordingly, alumina as the support for use in a heat-mode lithographic printing plate precursor is demanded to be improved in the low heat insulating property due to its high heat conductivity while maintaining easy applicability of various surface treatments and excellent properties such as hydrophilicity and dimensional precision stability of the aluminum.
In order to improve the low heat insulating property of the aluminum support, for example, a method of increasing the thickness of an anodic oxide film by utilizing the property such that the anodic oxide film formed on a lithographic printing plate support is by itself low in the heat conductivity, and a method of forming an anodic oxide film and then dipping the support in an aqueous alkali solution to enlarge the diameter of pores present in the film and thereby increase the porosity of the film have been proposed.
However, for increasing the thickness of the anodic oxide film, a large quantity of electricity is necessary at the time of forming the anodic oxide film and this gives rise to an increase in the cost. In the method of increasing the porosity of the film, the strength of the film decreases and therefore, when the film is scratched, an ink enters into the scratch to cause staining. That is, the method of providing an anodic oxide film has a problem in that both the film strength and the heat insulating property cannot be satisfied at the same time, more specifically, a sufficiently high film strength cannot be obtained and cost-up or staining is caused, though excellent heat insulating property may be obtained and the low sensitivity may be improved.
For example, Patent Document 1 (JP-A-2001-318458 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)) describes a technique for enhancing the heat insulating property on the support surface and thereby elevating the sensitivity of the produced heat-mode lithographic printing plate, where an anodic oxide film with a predetermined porosity and having micropores with a predetermined diameter is formed by controlling the conditions for anodization of an aluminum plate and applying treatments such as a treatment for enlarging the pore diameter of micropores of the oxide film after the anodization step and a pore-sealing treatment.
Also, Patent Document 2 (JP-A-2002-2133) describes a heat-sensitive lithographic printing plate, where a hydrophilic layer containing hollow particles is provided between the support and a heat-sensitive layer and thereby, enhancement of the heat insulating property and in turn, elevation of the sensitivity can be attained.
However, these techniques of enhancing the heat insulating property of the support of a heat-sensitive lithographic printing plate have a problem in that in order to increase the thickness of the oxide film, an extra quantity of electricity is required or the process is complicated and this leads to the increase in the production cost.
As a film for taking the place of the anodic oxide film formed on a lithographic printing plate support, for example, a hydrophilic layer for a lithographic printing plate has been proposed, which comprises a hydrophilic layer containing alumina particles and in which the hydrophilic layer is treated with a solution containing a silicic acid (see, Patent Document 3). Also, a method for producing a photosensitive substance has been proposed, which comprises a step of coating a slurry containing at least an inorganic non-metal particle and a monobasic phosphate on an aluminum surface and thoroughly dehydrating and drying it at a temperature of at least 230° C. or more to form a hydrophilic ceramic layer, and a step of forming an organic photosensitive layer on the hydrophilic ceramic layer (see, Patent Document 4).
However, the hydrophilic layer for a lithographic printing plate is a layer formed by utilizing the self-film-forming property of the alumina sol and the film strength is weak. Therefore, the hydrophilic layer and a lithographic printing plate support where the layer is provided are inferior in the scratch resistance and when a lithographic printing plate is produced, poor press life may result.
The lithographic printing plate having provided thereon the hydrophilic ceramic layer sometimes fails in having a sufficiently high staining resistance. Furthermore, this hydrophilic ceramic layer is formed through a drying step at a high temperature exceeding 230° C. and the drying equipment capable of performing such high-temperature drying is generally expensive. In addition, if dried at an excessively high temperature (for example, 260° C. or more), the aluminum plate where the hydrophilic ceramic layer is provided is softened to impair the excellent dimensional precision stability or the like of the aluminum plate and particularly, plate elongation sometimes occurs at the printing to cause a trouble that the substrate and the image come out of register.
In many cases, a lithographic printing plate obtained by using such a lithographic printing plate support where the hydrophilic layer for a lithographic printing plate or the hydrophilic ceramic layer is provided suffers from inferior press life and inferior staining resistance in the mass printing of producing a large number of printed matters. The improvement of these printing performances is demanded.
In performing the printing by using a lithographic printing plate, an operation of adjusting the amount of fountain solution (water amount) during printing is generally necessary. At this operation, when light is excessively reflected on the plate surface, the adjustment to a proper water amount becomes difficult and staining is sometimes generated. Therefore, the reflection of light must be suppressed to a certain degree or less on the surface of a lithographic printing plate support, which works out to a non-image area of the lithographic printing plate.
In both of the above-described lithographic printing plate supports, the reflection amount of light increases and when mounted on a press, the plate surface shines even with a small water amount. This phenomenon is called “shiny” and this is an undesired phenomenon from the standpoint of confirming the adjustment of water amount (suitability for plate inspection). Improvement of this phenomenon is also demanded.
Patent Document 1:JP-A-2001-318458Patent Document 2:JP-A-2002-2133Patent Document 3:JP-A-2000-169758Patent Document 4:U.S. Pat. No. 4,542,089